FIG. 1 is a flow diagram that illustrates filtering of a chemical solution in a conventional chemical solution circulation system. A chemical solution is circulated by suction of a pump 5 front an outer bath 3 of a chemical solution bath 1 and supplied to the primary side of a filter in a filtration unit 7 via the pump 5. Then, the chemical solution passes through the filter in the filtration unit 7 to reach the secondary side of the filter during which rubbish and particles in the chemical solution are retained by the filter. The chemical solution, freed of rubbish and particles, reaches an inner bath 2 of the chemical solution bath 1, where it acts on wafers (not shown) to etch or wash them.
FIG. 2 is a flow diagram that illustrates filtering in a chemical solution supply system, in which a chemical solution 4 is drawn by pump 5 from a chemical solution bottle 6 and supplied to a chemical solution bath I through a Filtration unit 7. Since rubbish and particles are retained by a filter in the filtration unit 7 the supplied chemical solution is cleaned. Alternatively, the chemical solution 4 may be pressure-fed by N.sub.2 gas or the like from the chemical solution bottle 6 to the filtration unit 7.
Chemical solution circulation system or chemical solution supply system often contain bubbles, air or the like (hereinafter collectively referred to as air) which are generated by the operation of the pump, N.sub.2 gas for pressure feeding, leakage at pipeline joints, the variation in the pipeline diameter or other factors. Thus, air in the chemical solution frequently reaches the primary side of the filter in the filtration unit 7.
Some air does not enter the filter pores 8 in the filtration unit 7, but rather coalesces into air particles 10 that gather at the top of the filtration unit 7 as shown in FIG. 3. These air particles are conventionally recycled to the outer bath 3 of the chemical solution bath 1 or the chemical solution bottle 6 by opening and closing a deaeration valve 11 provided in the deaeration line 9 connected to the top of the filtration unit 7 (FIGS. 1 and 2).
However, the remaining air is deposited on the membrane of filter 8 or is forced into the membrane of the filter 8 by the pump pressure or pressure feeding. The air forced into pores in the membrane of the filter 8 is stabilized there and gradually blocks the pores of the filter 8. Alternatively, the filter pores may also be blocked with rubbish, particles, etc.
Such blockage of the filters with air (airlock phenomenon) decreases the filtration flow rate of the circulating chemical solutions i.e. a decrease in the amount filtered per unit time. Therefore, the ability to remove particles from a chemical solution bath by filtration is reduced. These particles then adhere to products to constitute a major cause of low product yields. In extreme situations, no circulation takes place so that the resultant pressure damages the pump.
With chemical solution supply filters, the lowered filtration flow rate may lead to a longer supply period, and accordingly a longer solution replacement period in the system. In extreme cases, no chemical solution is supplied to the system.
Japanese Patent Application No. 75012/89 and Japanese Patent Application No. 127006/89 disclose examples of chemical solution filtration systems to preventing airlock phenomenon. The chemical solution filtration system disclosed in Japanese Patent Application No. 75012/89 comprises an air reservoir 23 connected via a connecting pipeline 24 to the primary stage of a filtration unit 13 that incorporates a filter membrane 14. The air reservoir 23 includes a deaeration line 19 and a liquid level sensor 20, as shown in FIG. 4.
In this system, a deaeration valve 22 provided in the deaeration line 19 is opened when the liquid level sensor 20 detects that the liquid level on the primary side of the filter has been lowered to a determined level by the air introduced from a chemical solution inlet 17. When the liquid level sensor 20 detects a rise in liquid level, on the contrary, the deaeration valve 22 is closed.
In this system, however, chemical solutions are pumped from the chemical solution inlet 17 through the air reservoir 23 and the connecting pipeline 24 to the primary side of the filter 14. Therefore, the air 12 that reaches the primary side of the filter 14 in the filtration Unit 13 or is generated from a bubble-forming chemical solution in the filtration unit 13 cannot not reach the air reservoir 23 through the connecting pipeline 24 because of the flow of the chemical solution.
The chemical solution filtration system, disclosed in Japanese Patent Application No. 127006/89, comprises a deaeration port 27 located at the top of a filtration unit 13. Filtration unit 13 incorporates a filter membrane 14, an air reservoir 23 and an air recycle line 15 connected between the deaeration port 27 and chemical solution inlet 17 of the air reservoir 23, as shown in FIG. 5. In this system, the air 12 that reaches filtration unit 13 or is generated in the filtration unit 13 flows in direction 18 through the air recycle line 15 to the air reservoir 23.
In this system however, chemical solutions can reach the filtration unit either by striking the baffle 16 or going along the air recycle line 15. Thus, it was found that the air 12 could not rise up against the flow of chemical solutions reaching the primary side of the filter membrane 14. The flow of chemical solutions prevented air 12 from escaping through the air recycle line 15 which considerably reduced the deaeration effect.
Therefore, neither of the above two systems could effectively overcome airlock phenomenon.